Copper post solder bumps on substrates

ABSTRACT

A method of assembling a semiconductor flip chip comprising a wafer having solderable electrical conducting sites and a substrate having electrical connecting pads and electrically conductive posts operatively associated with the pads and extending away from the pads to terminate in distal ends, comprises the pre-assembly steps of solder bumping the distal ends through openings in a solder mask by injection molding solder onto the distal ends so that the distal ends extend into the mask through the openings to produce a solder bumped substrate, and soldering the solder bumped substrate to the sites.

DESCRIPTION OF THE INVENTION

1. Field of the Invention

The field of the invention comprises electrical interconnects betweensemiconductor devices and substrates on which such devices are packaged,and more particularly, injection molded solder techniques used to makethese interconnects.

2. Background of the Invention

For solder flip chip assembly of a first level package or device, thesolder interconnects are formed by joining solder bumps on the chip withsolder bumps on the substrate. The solder bumps on the substrate helpcompensate for chip bump height variations and substrate warpage. Flipchip technology allows the largest numbers of inputs/outputs (“I/Os”)for the smallest footprint of the chip. This enables manufacturing smallpackages known in the art as chip-scale packages.

IBM introduced this technology in the early 1960's with the solid logictechnology in their IBM System/360™. It extended interconnectioncapabilities beyond existing wire-bonding techniques, allowing the areaarray solder-bump configuration to extend over the entire surface of thechip (die) providing solder bumps for interconnection to a substrate bythe so-called “C4” method. This allowed for the highest possible I/Ocounts to meet the increasing demand for electrical functionality andreliability in IC technology. “C4” comprises the term for describing themethod of connecting semiconductor and other devices, also known as theIBM “flip chip” or “controlled collapse chip connection,” from which theindustry derives the acronym “C4.” The devices that employ C4 technologycomprise integrated circuits (“IC” chips), passive filters, detectorarrays and microelectromechanical systems (“MEMS”) all of which are wellknown in the art. The present invention comprises processes formanufacturing these devices, and the products obtained by theseprocesses.

The C4 method interconnects devices to external circuitry by means ofsolder bumps that have been deposited on semiconductor chip pads orsubstrates. In order to mount the chip to external circuits such as acircuit board or another chip or a wafer used in manufacturing otherchips, a chip having solder bumps is flipped over so that the solder isaligned with matching connecting sites (e.g., connector pads) on anexternal circuit, and the connection completed by raising thetemperature of the solder so that it flows and adheres to the connectingsites.

The original wafer-bumping process of metal mask evaporation comprisedthe evaporation onto a wafer surface of solder through mask openings inan area array fashion. This wafer-bumping and the structure obtained aresometimes referred to as ball limiting metallurgy or under boardmetallization, under mask metallization, or under mask-bump metallurgy(“UBM”). In its broader aspect UBM comprises the application of a metalcoating to the die contact pads such as aluminum or copper contact pads,where the metal coating provides a surface that can adhere to solder.The UBM typically consist of an adhesion layer, such as Ti or TiW, abarrier layer, such as Ni and a solder wettable layer, such as Cu or Auto which the solder gets attached.

The need for increased I/O density and count, and pressures to lower thecost of flip-chip interconnections have spurred the development of otherwafer bumping techniques such as electroplating orstencil-printing/paste-screening (solder paste) bump processes. Some ofthe more newly developed bumping processes include transfer printing,solder jetting, and bumpless and conductive particle applications.

Other techniques used for the solder bumping process on thesemiconductor substrates include, for example, screen printing of solderpaste, ball mounting of preformed solder balls, injection molded solderand the like.

The overview of flip chip technology shows its major advantage lies inutilizing the total chip area to make the I/O connections, whereas wirebonding uses only the chip periphery.

For solder flip chip assembly at tight bump pitch in the first levelpackaging, the flip chip interconnects are formed by the connectionbetween solder bumps on the chip side and solder bumps on the substrateside. The solder bumps on the substrates help compensate for die bumpheight variations and substrate warpage. In order to get stringentsubstrate bump co-planarity, a coining process is applied to form flattops on the solder bumps.

The current manufacturing technology for forming solder bumps on organicsubstrates is the solder paste stencil printing method. The stencilprinting method is a low cost simple process for forming solder bumps onsubstrates which have island type I/Os without having to employ a photolithography process.

As an alternative to the bumps on the substrates, Tessera reportedetched Cu post substrate technology that can potentially reduce theinterconnection thermal resistance. Hongyu Ran et al, “Thermalcharacterization of copper contact interconnect for DRAM packagestacking in memory-intensive consumer applications”, AdvancingMicroelectronics, Vol. 34, No. 6 (2007), pp. 10-14.

The Cu posts on the substrate enable a higher stand-off height betweenthe chip and the substrate and better capability to carry higher currentwhen the current flows from the substrate to the chip. The Cu posts onthe substrate however do not include solder, so they need a sufficientvolume of solder from the bumps on the chips to enable flip chipassembly. Even if the Cu-post substrates are used for flip chip packingwith dies that have enough solder bumps, the increase of the Cu/solderratio in the interconnects increases the stress transmitted to theCu/ultra low k layers in the back-end-of-line (“BEOL”) structure andresults in flip chip assembly failure.

Also, for reliability benefits on electromigration to improve currentcarrying capability when the current flows from the chip to thesubstrate, Cu pillar die bumps only or Cu pillar die bumps with smallsolder caps have been integrated as chip side bumps in high volumemanufacturing.

Claims have been made that the integrated Cu die side bumps using a Cuelectroplating process in high volume manufacturing provide reliabilitybenefits with regard to stress, electromigration and thermalconductivity. Andrew Yeoh et al., “Copper die bumps (First LevelInterconnect) and Low-K dielectrics in 65 nm high volume manufacturing”,Proceedings of 56th Electronic Components and Technology Conference, p.1611, San Diego, Calif., May. 2006.

In the cases of Cu pillar die bumps only, or Cu pillar die bumps withsmall solder caps on the chip side, the Cu posts on the substratescannot be used for flip chip packaging. The conventional flip chipassembly with Cu pillar die bumps only with Cu posts on the substrate isnot possible because there is not any solder materials in theinterconnect. Also, in the case of Cu pillar die bumps with small soldercaps, the Cu/Sn ratio in a interconnect is too high.

Accordingly, it is generally desirable to have a new packaging paradigmfor flip chips with high current capability in both directions of thesolder joint, that is, from the chip to the substrate and from thesubstrate to the chip, along with less stress transmitted to the BEOLstructure in order to increase the reliability of electronic products,i.e. the aforementioned devices.

RELATED ART

Gruber, et al. U.S. Pat. No. 7,713,575 and US Patent ApplicationPublications US20090308308 and US20070272389 describe a compliant moldto deposit coplanar solder material interconnectors on a wafer. Itreceives solder depositions on a wafer's surface with either a one stepcoplanar deposition or subsequent coining of deposited solder to form acoplanar surface. The compliant mold has both a rigid side and acompliant side, which makes this method difficult to be applied toorganic substrates that have significant warpage (bending). Warpage oforganic substrates results in leakage of molten solder due tonon-contact between the mold and the organic substrate. Also, with thecavities of the mold having a straight wall and the diameter of cavitiesbeing almost the same as the pad size, a large yield loss is expecteddue to breaking and/or sticking of solders inside the cavities of themold.

Lin, US Patent Application Publication US20090079094 discloses a flipchip semiconductor package having a substrate with a plurality of activedevices. A contact pad is formed on the substrate in electrical contactwith the plurality of active devices. A passivation layer, secondbarrier layer, and adhesion layer are formed between the substrate andan intermediate conductive layer. The intermediate conductive layer isin electrical contact with the contact pad. A copper inner core pillaris formed by plating over the intermediate conductive layer. The innercore pillar has a rectangular, cylindrical, toroidal, or hollow cylinderform factor. A solder bump is formed around the inner core pillar byplating solder material and reflowing the solder material to form thesolder bump. A first barrier layer and wetting layer are formed betweenthe inner core pillar and solder bump. The solder bump is in electricalcontact with the intermediate conductive layer.

Photolithography and electroplating cannot be used for solder bumping onthe organic substrate without an additional seed layer in theelectroplating step because the pads on the substrate are notelectrically connected.

Chang, et al., US Patent Application Publication US20080296764 describesan enhanced wafer level chip scale packaging (WLCSP) copper electrodepost having one or more pins that protrude from the top of the electrodepost. When the solder ball is soldered onto the post, the pins areencapsulated within the solder material. The pins not only add shearstrength to the soldered joint between the solder ball and the electrodepost but also create a more reliable electrical connection due to theincreased surface area between the electrode post/pin combination andthe solder ball. Moreover, creating an irregularly shaped solder jointretards the propagation of cracks that may form in the intermetalcompounds (IMC) layer formed at the solder joint. The step of bumping onthe wafers (300) requires electroplating the copper and ball mountingthe preformed solder balls.

Alvarez, US Patent Application Publication US20040130034 uses a layer ofgold (405) disposed on upper surfaces (225) of copper pillars (210) on abumped wafer (205) for forming a wafer level chip scale package. Coatingmaterial (410) is then applied to a level which is less than the heightof the copper pillars (210), and etchant is disposed to remove coatingmaterial on the layer of gold (405) and to remove coating material (410)adhering to side surfaces of the copper pillars (210). Solder depositsare then disposed on the gold layer and reflowed to form balls (405) onthe ends of the copper pillars (210), with the copper pillars (210)protruding into the solder balls (405). Bumping on the wafers (205) isthereby effected by electroplating the copper and ball mountingpreformed solder balls.

Watanabe, U.S. Pat. No. 7,626,271 describes a method for producing asemiconductor device that employs a step in FIG. 4(B), of using asealing portion 44 to cover exposed wiring patterns 42, base metalpatterns 36, post electrodes 46, and interlayer insulation layer 34followed by applying a sealing resin to fill in recess portions 42 bb ofthe post electrode mounting portions 42 b of wiring patterns 42. Thisexposes lower portions of the bottom surfaces 46 b of the postelectrodes 46. The sealing portion 44 may be formed using a well-knownsealing material such as an epoxy type mold resin with a well-knownmethod in the step of forming the sealing portion 44. After the sealingresin covers the top surfaces 46 a of the post electrodes 46, thesealing resin is ground from a front side thereof with a well-knowngrinding method or a polishing method, so that the top surfaces 46 a ofthe post electrodes 46 are exposed. In the next step, as shown in FIG.4(B), solder balls 48 a as the outer terminals 48 are formed on the topsurfaces 46 a of the post electrodes 46 exposed from a flat surface ofthe sealing portion 44. The process appears to use preformed solderballs for applying solder balls to the electroplated post electrode (46)of chip (30).

Fjelstad, U.S. Pat. No. 7,528,008 describes die pillar structures with(1), is replaced with a plurality of metallic portions 210 of a geometrysimilar to the photoresist portions (180/190/200) in FIGS. 3A-C.metallic portions 210 consisting of an etch resistant metal, such asnickel. A conductive layer around the metallic portions 210 may then beetched leaving the post capped with a conductive top. This conductivetop may then be plated with a highly conductive layer, such as gold or agold alloy. This conductive top further increases the reliability of anelectrical connection when the posts are inserted into the type ofsocket shown in FIG. 4A. In an alternate embodiment, electro depositionsolder can also be used as an etch resist. After the posts are created,the solder can then be reflowed to create a solder coated post. If thesolder is reflowed after the post has been inserted into a test socket,it will create a more permanent electrical connection with the socket.

Lin, et al., U.S. Pat. No. 7,446,419 discloses a semiconductor chiphaving a welded stack of metal balls. For instance, a metal ball can bea stud bump that includes a ball bond and a stump and consistessentially of a ball bond, or alternatively, a ball bond without astump. Furthermore, the metal balls can each be gold, aluminum, copperor solder, or alternatively, a solder coating and a coated metal,wherein the solder coating contacts the encapsulant and the coatedmetal, and the coated metal is spaced from the encapsulant.

Tan, et al., U.S. Pat. No. 7,462,942 describes the method of formingsolder/Cu pillar bumps on wafers by electroplating. The electroplatingcannot form bumps on the organic substrate without additional metal seedlayer and photolithography because the pads on the organic substrate arenot electrically connected. (Col 2:59) Pillar metal layer 26 islead-free and is preferably comprised of copper (Cu). An optional layerof solder 28 is formed/plated over Cu pillar layer 26.

Chen, et al., U.S. Pat. No. 7,476,564 describes a copper pillar on awafer; forming a solder on a substrate; and covering substantially allof the external surfaces of the pillar with the solder.

Lin, et al., U.S. Pat. No. 7,271,483 uses a bump structure on asemiconductor package for connecting a semiconductor element to acarrier of a semiconductor package. The semiconductor element has atleast one electrical connection pad on its surface. The bump structureincludes a UBM layer formed on an electrical connection pad and anI-shaped conductive pillar disposed on the UBM layer, wherein a middleportion of the conductive pillar has a width smaller than that of anupper end and a lower end of the conductive pillar respectively. Theconductive pillar also has a solder material applied to it. Lin, et al.use photolithography to electroplate the solder onto the copper pillarusing the metal seed layer as a conductor in the process.

Knapp, et al., U.S. Pat. No. 6,835,580 describes the ball mountingmethod of preformed solder balls to form solder bumps. This consists offorming a direct chip attach (DCA) device (1) includes attaching a chip(3) to a lead frame (2). Conductive studs (22) are attached to bondingpads (13) on the chip (3) and a flag (18) on lead frame (2). The chip(3) and flag (18) are enclosed with an encapsulating layer (4), andopenings (6) are formed in an upper surface (7) to expose conductivestuds (22). In one embodiment, a masking layer (51) is applied to thelead frame (2), and the structure is then placed in an electrolessplating apparatus (61). While in the plating apparatus (61), aninjection device (66) injects plating solution (71) towards the uppersurface (7) and openings (6) to enhance the formation of barrier layers(24) on the conductive studs (22). Solder bumps (9) are then attached tobarrier layers (24) through openings (6).

Hwee, et al., U.S. Pat. No. 6,510,976 describes the ball mounting methodusing preformed solder balls to form solder bumps on copper posts on thepads of the wafer. An oxidized (220) copper leadframe and asemiconductor die with copper posts extending from die pads, and withsolder balls coated (225) with flux on the end of the copper posts, areprovided. The semiconductor die is placed (230) on the oxidized copperleadframe, with the solder balls abutting portions of the layer ofoxide, above and aligned with, interconnect locations on the leadframe.When reflowed (235), the flux on the abutting portions of the oxidelayer selectively cleans these portions of the oxide layer, away fromthe interconnect locations. In addition, the solder balls change tomolten state and adhere to the cleaned copper surfaces at theinterconnect locations. Advantageously, the rest of the oxide layer thatis not cleaned away provides a passivation layer that advantageouslycontains and prevents the molten solder from flowing away from theinterconnect locations.

Lin, U.S. Pat. No. 6,103,552 describes electroless plating, screen orstencil printing methods to form solder bumps on copper posts on theredistribution layers of a wafer. This involves a WLP process includinga post passivation RDL. The RDL is supported on a layer of polymericmaterial that is deposited on the passivation layer of the semiconductorstructure. Another polymeric layer is deposited over the RDL, and etchedor drilled to provide a via for over-filling with a metal to form aninterconnect (i.e., a conducting post) that extends above and beyond theopening of the via. The top polymeric layer and the bottom polymericlayer are separated by a layer of chrome-copper, and therefore do nottouch between the RDL structures. A solder bump attached to theprotruding end of the post is formed by electroless plating, screen orstencil printing.

YOR920080772US1, which is commonly owned by the same assignee as theassignee of the present invention, discloses a method and apparatus forforming solder bumps on organic substrates, whereby molten solder isinjected into a mask which is aligned on a substrate.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides structures, articles ofmanufacture and processes that address these needs to not only provideadvantages over the related art, but also to substantially obviate oneor more of the foregoing and other limitations and disadvantages of therelated art such as providing electromigration-resistant under-bumpmetallization. Not only do the written description, claims, abstract ofthe disclosure, and the drawings that follow set forth various features,objectives, and advantages of the invention and how they may be realizedand obtained, but these features, objectives, and advantages will alsobecome apparent by practicing the invention.

To achieve these and other advantages, and in accordance with thepurpose of the invention as embodied and broadly described herein, theinvention comprises a process, composition, and an improved structure.To achieve these and other advantages, and in accordance with thepurpose of the invention as embodied and broadly described herein, theinvention comprises an article of manufacture and a process comprising:

-   -   1. Forming solder caps or bumps on conductive pillars in a flip        chip assembly, where the pillars comprise metallic posts such as        Cu posts, by using injection molded solder (IMS) technology to        solder bump or coat the distal ends of the posts, where the chip        comprises a wafer and a substrate and the posts extend from the        substrate.    -   2. bumping the distal ends through a mask and the distal ends        extend from the substrate into the mask;    -   3. forming high volume IMS solder bumps on the distal ends of        the Cu posts on the substrate to reduce the size and volume of        the solder bumps on the wafer or eliminate the bumps on the        wafer.    -   3. fabrication of selectively different bump structures by        combining solder only and solder with a Cu post on one substrate        to inter alia decrease the stress in Cu/ultra low k layers.    -   4. fabrication of small height Cu posts on the substrate        combined with the injection molded solder method;    -   5. in a further aspect, forming a semiconductor flip chip from a        wafer having solderable electrical conducting sites and a        substrate having electrical connecting pads and electrically        conductive posts operatively associated with the pads and        extending away from the pads to terminate in distal ends, by the        steps comprising solder bumping the distal ends through openings        in a solder mask by injection molding solder onto the distal        ends to produce a solder bumped substrate and soldering the        solder bumped substrate to the sites where the distal ends        extend into the mask;    -   6. employing the openings in the mask as reservoirs for        receiving molten solder wherein the reservoir and the posts        sealingly engage one another toward the proximal ends of the        post in a manner to substantially minimize or eliminate the        leakage of molten solder from the reservoir. In one embodiment        we achieve this function by employing posts that taper from the        proximal ends toward the distal ends in a narrowing fashion.        This can also be achieved by employing reservoirs that taper        from the point of solder injection toward the other end of the        reservoir in a narrowing fashion, Also, both tapered posts and        tapered reservoirs can be employed. An advantage in using        tapered posts comprises easier insertion and/or alignment of the        posts in the reservoir.

The “small height” Cu posts refers to Cu post heights shorter than thesolder resist surface of the organic substrate. Generally, the solderresist has a height of around 20 microns. In flip chip technology,greater than about 25 microns height over solder resist is required forgood assembly yield. Therefore, small height Cu posts means from about 5microns to about 20 microns which is shorter than the height of thesolder resist. And, IMS adds solder on the Cu post to make the combinedheight of the Cu post and the solder added to the Cu post about 25microns over the solder resist surface.

Small height Cu posts decrease the Cu/Sn ratio in an interconnect(joint) and decrease the stress in the BEOL, but, small height Cu postsmakes low stand off height (the gap between a die and a substrate).“Small height Cu posts” is not to be confused with “stand off heights”which is the height of the Cu post plus the height of the solder bump.The disadvantages of low stand off heights are:

-   -   (1) it is difficult to clean flux residue;    -   (2) underfill flow is not easy, so voids are formed; and    -   (3) high stress from the CTE (Coefficient of thermal expansion)        mismatch between the chip or wafer and the organic substrate.

The present invention, however, overcomes the difficulties of low standoff heights by placing injection molded solder on Cu posts to producehigh bumps on substrates and helps to maintain high stand off heightsafter flip chip assembly which in turn provides the benefit of not onlyfurther decreasing the stress on the resultant structure, but alsoproviding a structure where it is easy to clean the flux residue,underfill flow is easy thereby eliminating or minimizing void formationcompared to prior processes, and minimizing or eliminating high stressesfrom the CTE mismatch between the chip or wafer and the organicsubstrate.

A core idea of this invention comprises using the combination of Cu postand molten solder injection with a mask to simultaneously form on onesubstrate, solder bumps having different compositions such as solderbumps comprising Sn, In, SnIn, SnCu, SnAg, SnAgCu, SnBi, SnPb, SnZn,SnSb, AuSn, SnAgCuZn, SnAgCuBi and alloys thereof and mixtures thereofand the like.

The injection of molten solder with a mask can form the solder bumpswith uniform height on substrates independent of the shape any featuresinside the holes or reservoirs in the mask.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying Figures, together with the detailed description hereinserve to further illustrate various embodiments and to explain variousprinciples and advantages of the present invention.

FIG. 1 is a side elevation in cross-section showing Tessera's Cu postsubstrates. All pads on a substrate have etched Cu posts and there is nosolder on the substrate.

FIGS. 2A, 2B, 2C, 2D, comprising one embodiment of the invention,illustrate a side elevation in cross-section showing the formation ofsolder caps on Cu posts on a substrate by using the method described inYOR920080772US1.

FIG. 3, comprising one embodiment of the invention, illustrates a sideelevation in cross-section that depicts a method of flip chip assemblybetween a chip with small solder bumps and a substrate with the highvolume solders on Cu posts which are formed on pads by using the processflow described by FIGS. 2A, 2B, 2C, 2D.

FIG. 4, comprising one embodiment of the invention, illustrates a sideelevation in cross-section that illustrates the method of flip chipassembly between a chip with small Cu pedestals and a substrate by usinghigh volume solders on Cu posts which are formed on pads by employingthe process flow explained in FIGS. 2A, 2B, 2C, 2D.

FIG. 5, comprising one embodiment of the invention, illustrates a sideelevation in cross-section that depicts methods of flip chip assemblybetween a chip without bumps and a substrate with the high volumesolders on Cu posts which are formed on the pads by using the processflow which is explained in FIGS. 2A, 2B, 2C, 2D.

FIGS. 6A, 6B, 6C, 6D, comprising one embodiment of the invention,illustrates a side elevation in cross-section that depicts methods offorming uniform solder bump heights when Cu posts are selectively formedin one substrate. When the Cu posts are selectively formed, the totalstress is reduced during the flip chip assembly process because thenumber of Cu posts in a substrate decreases. Injection molded solderbumping is a unique method for forming uniform solder bump heights whenthe Cu posts are selectively formed in one substrate.

FIG. 7 comprising one embodiment of the invention, illustrates thatother solder bumping methods such as micro-ball mounting cannot formuniform solder bump heights when Cu posts are selectively formed in onesubstrate.

FIG. 8 comprising one embodiment of the invention, illustrates a sideelevation in cross-section that depicts methods of flip chip assemblybetween a chip with small solder bumps and a substrate with high volumesolders on selectively formed Cu posts on a substrate by using theprocess flow which is explained in FIGS. 6A, 6B, 6C, 6D.

FIG. 9 comprising one embodiment of the invention, illustrates a sideelevation in cross-section that depicts methods of assembly of a flipchip with small Cu pedestals for joining to a substrate by means of highvolume solders on selectively formed Cu posts on the substrate by aprocess explained in FIGS. 6A, 6B, 6C, 6D.

FIG. 10 comprising one embodiment of the invention, illustrates a sideelevation in cross-section that depicts methods of flip chip assemblybetween a chip without bumps and a substrate with the high volumesolders on selectively formed Cu posts on a substrate by using theprocess flow which is explained in FIGS. 6A, 6B, 6C, 6D.

FIGS. 11A, 11B, 11C, 11D comprising one embodiment of the invention,illustrates a side elevation in cross-section of a low cost processwherein stud bumps from wire bonding can be used for forming selectivemetal bumps on a substrate. Stud bumps comprising Cu, Au, and Al oralloys thereof or any combination of Cu, Au, and Al or alloys thereofcan be used in this method.

FIGS. 12A, 12B, 12C comprising one embodiment of the invention,illustrates a side elevation in cross-section of selective Cu posts withsolder bumps on a substrate where the posts function as spacers for aflip chip with wafer level underfill (FIG. 12A), with no-flow underfill(FIGS. 12B), and 3D stacked chips (FIG. 12C) to avoid the collapse ofjoints. US Patent Application No. 2009/0108472 explains the wafer levelunderfill process. U.S. Pat. No. 7,087,485 B2 explains the no-flowunderfill process.

FIGS. 13A, 13B, 13C, 13D, 13E, 13F comprising one embodiment of theinvention, illustrates a side elevation in cross-section of a smallheight Cu posts on the substrates to facilitate mask alignment forinjection molded solder. The top of Cu post can be at the same level asthe solder mask or lower than the solder mask. The small height of Cuposts can decrease the Cu/solder ratio in one joint and decrease thestress on the back end of the line during flip chip assembly processes.

FIGS. 14A, 14B, 14C, 14D, 14E, 14F comprising one embodiment of theinvention, illustrates a side elevation in cross-section of analternative FIG. 13A, 13B, 13C, 13D, 13E, 13F of forming uniform solderbump heights when the small Cu posts are selectively formed in onesubstrate.

DETAILED DESCRIPTION OF THE INVENTION

To achieve these and other advantages, and in accordance with thepurpose of this invention as embodied and broadly described herein, thefollowing detailed embodiments comprise disclosed examples that can beembodied in various forms.

The specific processes, compounds, compositions, and structural detailsset out herein not only comprise a basis for the claims and a basis forteaching one skilled in the art to employ the present invention in anynovel and useful way, but also provide a description of how to make anduse this invention.

The present invention comprises methods of forming solder bumps on Cuposts positioned on the organic substrate of a semiconductor/organicsubstrate device by means of injection molding of molten solder througha reusable mask positioned on the device. We refer to semiconductors,wafers, dies and chips in our description of the invention and intendthat these terms are to be considered as interchangeable, as are theterms copper posts and copper pillars.

An advantage of the invention comprises formation of solder bumps on Cuposts without an additional metal seed layer and without the need ofemploying a photolithographic process. The invention also comprisesmethods for forming uniform bump heights when the Cu posts areselectively formed in one substrate. When the Cu posts are selectivelyformed for power joints, the total stress on low k layers in the die isreduced during the flip chip assembly process because the number of Cuposts in the substrate decreases. The uniformity of bump heights in onesubstrate when the Cu posts are selectively applied is very important toobtain relatively high assembly yield.

The injection molded solder bumping method is a unique method forforming uniform bump heights when the Cu posts are selectively formed inone substrate.

For the most part, the related art describes methods for forming astructure of solder on the copper pillars that extend from the pads ofthe substrate by using an electroplating method for both solder andcopper pillar. The electroplating method however, cannot form bumps onthe organic substrate without applying an additional metal seed layerfollowed by photolithography because the pads on the organic substrateare not electrically connected.

Some related art describes the direct attachment of preformed solderballs on copper pillars, i.e., copper posts, positioned on the waferpads. This method is very difficult to use with fine pitch copperpillars because of bridging issues with the solder bumps and throughputissues.

If the copper post on the substrate is higher than the solder mask, ballmounting with a mask also makes it very difficult to eliminate bridgingof the solder balls because the solder balls could slip down from thetop of the copper post even though a tacky flux is used.

FIG. 1 illustrates the prior art device of Ran et al. (supra) consistingof a wafer or die 102 having a metal contact pad 104 with a solder ball106 soldered to contact pad 104. Solder ball 106 in turn is soldered ina conventional way to a Cu post 108 that extends outwardly from anorganic substrate 110. 108 a comprises a three dimensional view of anetched Cu post 108 according to Ran et al. (supra). The “C4” padsreferred to in FIG. 1 comprise pads 109 on the organic substrate and 104on the chip or wafer, i.e., both 109 and 104 are called C4 pads.

FIGS. 2A, 2B, 2C, 2D illustrate one aspect of the invention comprisingmethods of solder bumping and structures obtained wherein Cu posts 208are positioned on an organic substrate 210 and project outwardly fromelectrically conductive pads 204 through solder resist_212 sometimesreferred to as a solder mask. The solder resist (212) prevents solderfrom bridging between conductors and creating short circuits.

Cu posts 208 extend through openings in mask 214 which allows placingmolten solder caps 216 on Cu posts 208 by means of an IMS process bymeans 218, an IMS (Injection molded solder) head. Solder is meltedinside IMS head 218 and the molten solder injected from head 218 intoholes of mask. Upon cooling of the solder to form solidified solder caps220, mask 214 is separated to provide solder bumped Cu posts having goodco-planarity. Good co-planarity cannot be achieved with the prior artball mounting process.

FIG. 3 illustrates one aspect of the invention comprising methods ofwafer bumping and structures obtained wherein the size of the solderbumps on the wafer can be significantly reduced for attachment to thesolder bumped Cu posts of the present invention. In FIG. 3, a solderbumped wafer or Si chip 302 includes electrical contact pads 305, solderbumped with solder balls 306 (sometimes referred to as bumping solder),positioned to face IMS solder bumped Cu posts 308 having solidifiedsolder 320 extending toward solder balls 306. Layer 303 comprises apassivation layer made from SiO2, Si3N4, or polyimide to prevent solderbridging. Cu posts 308 project outwardly from electrically conductivepads 304 through solder resist 312 onto organic substrate 310. Joiningwafer 302 to substrate 310 by soldering solder balls 306 to solder 320results in the formation of solder connectors 322 with the size of thebumping solder substantially reduced compared to structures in which thechip or wafer is solder bumped in a conventional way. This reduces thethickness of the structure thereby allowing assembly of multiple devicesin smaller packages.

FIG. 4 illustrates one aspect of the invention comprising methods ofsolder bumping and structures obtained wherein the solder bumps on thewafer are replaced with Cu pads 405 for attachment to the solder bumpedCu posts 408 of the present invention. In FIG. 4, a wafer or Si chip 402includes electrical contact pads 405 connected to or operativelyassociated with Cu posts 408 where Cu posts 408 are positioned to faceIMS solder bumped Cu posts 408 having solidified solder 420 extendingtoward Cu posts 408. Layer 403 comprises a passivation layer. Cu posts408 project outwardly from electrically conductive pads 304 on organicsubstrate 410 through solder resist 412. Joining wafer 402 to substrate410 by soldering Cu pedestals 406 to solder 420 results in the formationof solder connectors 422 to thereby reduce the thickness of theresultant structure compared to structures in which the wafer is solderbumped in a conventional way. This reduced thickness also allowsassembly of multiple devices in smaller packages.

FIG. 5 illustrates one aspect of the invention comprising methods ofsolder bumping and structures obtained wherein the solder bumps on thewafer are replaced with UBM pads 505 for attachment to the solder bumpedCu posts of the present invention. In FIG. 5, a wafer or Si chip 502includes UBM pads 505 connected to or operatively associated with Cuposts 508 where Cu pads 506 are positioned to face IMS solder bumped Cuposts 508 having solidified solder 520 extending toward UBM pads 503comprising a passivation layer. Cu posts 508 project through solderresist 512 from metal contact pads 504 positioned on organic substrate510. Joining wafer 502 to substrate 510 by soldering UBM pads 505 tosolder 520 results in the formation of solder connectors 522 to therebyreduce the thickness of the resultant structure compared to structuresin which the wafer is solder bumped in a conventional way. This reducedthickness allows assembly of multiple devices in smaller packages.

FIGS. 6A, 6B, 6C, 6D illustrates one aspect of the invention comprisingmethods of solder bumping and structures obtained wherein selective Cuposts 608 are positioned on an organic substrate 610 and projectoutwardly from electrically conductive pads 604 through solderresist_612. Cu posts 608 extend through openings in mask 614 whichallows placing molten solder by means of an IMS process comprisingmolten solder caps 616 on Cu posts 608 and molten solder 615 onelectrical conducting pads 604 that do not have Cu posts on them. Means618 comprises an IMS head.

In this embodiment, not all electrically conducting pads 604 have Cuposts 608 mounted on them; only alternate pads 604 include the Cu posts608. In alternate embodiments, different patterns of pads 604 free of Cuposts 608 can be positioned on inorganic substrate 610 suited to theneeds and design of the device ultimately assembled according to theprocess of the invention. Similarly, different patterns of pads 604having Cu posts 608 can be positioned on inorganic substrate 610, again,suited to the needs and design of the device ultimately assembledaccording to the process of the invention. In any event, we selectivelyplace Cu posts 608 on only some of the pads 604 in order to providenegative power interconnects to help reduce or eliminateelectromigration. We have found that reducing the number of Cu posts onthe substrate reduces the total stress on BEOL during the flip chipassembly process thereby reducing the failure rate in joining wafers toorganic substrates.

Upon cooling the molten solder 615 and 616 to form solidified soldercaps 620 and solidified solder columns 623, mask 614 is separated toprovide solder bumped Cu posts and solder columns having goodco-planarity. Good co-planarity cannot be achieved with the prior artball mounting process.

FIG. 7 illustrates an advantage of the invention compared to the priorart solder bumping method. In FIG. 7, selective Cu posts 7088 arepositioned on metal pads 7004 mounted on an organic substrate 7010 andproject outwardly from electrically conductive pads 7004 through solderresist 7012. Cu posts 7008 extend outwardly from organic substrate 7010which allows placing solder on them by means of a conventional ballmounting method as well as pads 7004 that do not have Cu posts on them.

The formed solder caps 7020 and solder bumps 7023 do not have goodco-planarity as can be seen by comparing FIG. 7 to FIGS. 6A, 6B, 6C, 6D.This illustrates that the ball mounting method cannot produce uniformsolder height on selectively formed Cu posts, which only the IMS methodof the present invention can provide.

FIGS. 8-14 illustrate other embodiments of the invention. The structuresand components of FIGS. 8-14 are identified therein or the structures orcomponents have been illustrated, identified, and described with regardto FIGS. 2A, 2B, 2C, 2D-FIGS. 6A, 6B, 6C, 6D.

FIGS. 8, 9, and 10 illustrate aspects of the invention comprisingmethods of solder bumping and structures obtained and comprise acombination of the elements of FIGS. 3, 4, 5 and FIGS. 6A, 6B, 6C, 6D toobtain structures with Cu posts 808, 908, and 1008, capped with solder820, 920 and 1020. FIGS. 8, 9, and 10 also illustrate solder columns823, 923, and 1023, substantially co-planar with solder caps 820, 920,and 1020. These structures join wafers, e.g., silicon chips 802, 902,and 1002 to inorganic substrates 810, 910 and 1010 by soldering to formsolder connector structures 822, 825, 922, 925, and 1022, 1025. Thisprovides selective Cu posts on a substrate that improve electromigrationreliability or higher stand-off in the resultant structures.

FIGS. 11A, 11B, 11C, 11D illustrate aspects of the invention comprisingmethods of solder bumping and structures obtained and comprise acombination of the elements of FIG. 10 but employs stud bumps 1108 madeof Cu, Au or Al wire or any combination thereof or any alloy thereof inlieu of Cu posts. The stud bumps are formed by a wire bonding processknown in the art and are used in this aspect of the invention to formselective metal bumps on an organic substrate as a low cost process.

Stud bumps 1108 are positioned on an organic substrate 1110 and projectoutwardly from electrically conductive pads 1104 through solder resist1112. Stud bumps 1108 extend through openings in mask 1114 which allowsplacing molten solder by means of an IMS process comprising moltensolder caps 1116 on Stud bumps 1108 and molten solder 1115 on pads 1104that do not have Stud bumps on them. Means 1118 comprises an IMS head.

In this embodiment, not all electrically conducting pads 1104 have Studbumps 1108 mounted on them; only alternate pads 1104 include the Studbumps 1108. In other embodiments, different pads 1104 free of Stud bumps1108 can be positioned on inorganic substrate 1110 suited to the needsand design of the device ultimately assembled according to the processof the invention. Similarly, different patterns of pads 1104 having Studbumps 1108 can be positioned on inorganic substrate 1110, again, suitedto the needs and design of the device ultimately assembled according tothe process of the invention. In any event, we selectively place Studbumps 1108 on only some of the pads 1104 in order to provide negativepower interconnects to help reduce or eliminate electromigration. Wehave found that reducing the number of Stud bumps on the substratereduces the total stress on BEOL during the flip chip assembly processthereby reducing the failure rate in joining wafers to organicsubstrates.

Upon cooling the molten solder 1115 and 1116 to form solidified soldercaps 1120 and solidified solder columns 1123, mask 1114 is separated toprovide solder bumped Stud bumps and solder columns having goodco-planarity.

FIG. 12A illustrates aspects of the invention comprising methods ofsolder bumping and structures obtained which are selective Cu posts withsolder bumps on an organic substrate that work as spacers for a flipchip with wafer level underfill. FIG. 12A illustrates a combination ofthe elements of FIGS. 3, 4, 5 and 6. Selective Cu posts on a substrate1210 employed in combination with solder bumps 1206 work as spacers forflip chips with wafer underfill 1230. US Published Patent Application2009/0108472 explains wafer underfill processes and compositions. Inthis aspect, solder columns 1223, substantially co-planar with soldercaps 1220, are employed with these solder caps 1220 for joining a wafer,e.g., silicon chip 1202 having solder balls 1206, to inorganic substrate1210, through solder structures 1220-1223 to provide these selective Cuposts. The joining is effected by thermal compression bonding whichcauses underfill 1230 to flow and bond to both the wafer or silicon chip1202 and the organic substrate 1210 as well as melting solder structures1220 and 1223 and solder balls 1206 to form solder connectors 1222 and1225.

FIG. 12B illustrates aspects of the invention comprising methods ofsolder bumping and structures obtained which are selective Cu posts withsolder bumps on an organic substrate that work as spacers for a flipchip and incorporates the method employed and the structure obtained inFIG. 12A except for the substitution of no-flow underfill 1232 for waferunderfill 1230. U.S. Pat. No. 7,087,485 explains no-flow unfillprocesses and compositions.

FIG. 12C illustrates aspects of the invention comprising methods ofsolder bumping and structures obtained which are selective Cu posts withsolder bumps on an organic substrate that work as spacers for a flipchip and incorporates the method employed and the structure obtained inFIG. 12A except for the substitution of 3D stacked chips 1203 for Sichip 1202 and the elimination of wafer level underfill 1230.

In the foregoing embodiments, not all electrically conducting pads 1204have Cu posts 1208 mounted on them; only alternate pads 1204 include theCu posts 1208. In alternate embodiments, different pads 1204 free of Cuposts 1208 can be positioned on inorganic substrate 1210 suited to theneeds and design of the device ultimately assembled according to theprocess of the invention. Similarly, different patterns of pads 1204having Cu posts 1208 can be positioned on inorganic substrate 1210,again, suited to the needs and design of the device ultimately assembledaccording to the process of the invention. In any event, we selectivelyplace Cu posts 1208 on only some of the pads 1204 in order to providenegative power interconnects to help reduce or eliminateelectromigration. We have found that reducing the number of Cu posts onthe substrate reduces the total stress on BEOL during the flip chipassembly process thereby reducing the failure rate in joining wafers toorganic substrates.

FIGS. 13A, 13B, 13C, 13D, 13E, 13F illustrate one aspect of theinvention comprising methods of solder bumping and structures obtainedto address issues where mask alignment becomes problematic. In thisaspect of the invention we adjust the height of the Cu posts to be aboutthe same height as the mask through which it projects or somewhat lessthan the height of the mask.

FIGS. 13A, 13B, 13C, 13D, 13E, 13F illustrate a combination of theelements of FIGS. 4 and 6A, 6B, 6C, 6D to obtain a structure with Cuposts 1308 positioned on an organic substrate 1310 and project outwardlyfrom electrically conductive pads 1304 through solder resist 1312. Cuposts 1308 extend up to or slightly below the openings in mask 1314which allows placing molten solder by means of an IMS process comprisingmolten solder caps 1316 on Cu posts 1308 that extend from electricalconducting pads 1304. Means 1318 comprises an IMS head

Upon cooling the molten solder 1315 to form solidified solder caps 1320,mask 1314 is separated to provide solder bumped Cu posts having goodco-planarity. Good co-planarity cannot be achieved with the prior artball mounting process.

Solder caps 1320 join wafers, e.g., silicon chips 1302, to inorganicsubstrate 1310, by soldering to form solder connector structures 1322.This provides Cu posts on a substrate that provides low stress duringassembly because of the reduced Cu height. Furthermore, Cu on thesubstrate improves electromigration reliability or “negative flow,”i.e., electrons flow from the substrate to the wafer or chip in a joint.Cu on the substrate also can improve interconnect thermal resistance. orhigher stand-off in the resultant structures.

FIGS. 14A, 14B, 14C, 14D, 14E, 14F illustrates a combination of theelements of FIG. 4 and FIGS. 6A, 6B, 6C, 6D to obtain a structure withCu posts 1408 positioned on an organic substrate 1410 and projectoutwardly from electrically conductive pads 604 through solder resist1412. Cu posts 1408 extend up to or slightly below the openings in mask1414 which allows placing molten solder by means of an IMS processcomprising molten solder caps 1416 on Cu posts 1408 that extend fromelectrical conducting pads 1404 and molten solder 1415 on pads 1404 thatdo not have Cu posts on them. Means 1418 comprises an IMS head.

In this embodiment, not all electrically conducting pads 1404 have Cuposts 1408 mounted on them; only alternate pads 1404 include the Cuposts 1408. In alternate embodiments, different patterns of pads 1404free of Cu posts 1408 can be positioned on inorganic substrate 1410suited to the needs and design of the device ultimately assembledaccording to the process of the invention. Similarly, different patternsof pads 1404 having Cu posts 1408 can be positioned on inorganicsubstrate 1410, again, suited to the needs and design of the deviceultimately assembled according to the process of the invention. In anyevent, we selectively place Cu posts 1408 on only some of the pads 1404in order to provide negative power interconnects to help reduce oreliminate electromigration. We have found that reducing the number of Cuposts on the substrate reduces the total stress on BEOL during the flipchip assembly process thereby reducing the failure rate in joiningwafers to organic substrates.

Upon cooling the molten solder 1415 and 1416 to form solidified soldercaps 1420 and solidified solder columns 1423. Mask 1114 is separated toprovide solder bumped Stud bumps and solder columns having goodco-planarity. Good co-planarity cannot be achieved with the prior artball mounting process.

Solder caps 1420 and solidified solder columns 1423 join wafers, e.g.,silicon chips 1402, to inorganic substrate 1410, by soldering to formsolder connector structures 1422 and 1425. This provides Cu posts on asubstrate that provides low stress during assembly because of thereduced Cu height. Furthermore, Cu on the substrate side improveselectromigration reliability or higher stand-off in the resultantstructures as well as interconnect thermal resistance. The Cu post withsolder is used in one embodiment as a power joint to provide electricalpower to the structure obtained, and the solidified solder columns 1423when soldered to the substrate and the wafer provide a signal jointbetween the substrate and the wafer.

In this embodiment, not all electrically conducting pads 1404 have Cuposts 1408 mounted on them; only alternate pads 1404 include the Cuposts 1408. In alternate embodiments, different patterns of pads 1404free of Cu posts 1408 can be positioned on inorganic substrate 1410suited to the needs and design of the device ultimately assembledaccording to the process of the invention. Similarly, different patternsof pads 1404 having Cu posts 1408 can be positioned on inorganicsubstrate 1410, again, suited to the needs and design of the deviceultimately assembled according to the process of the invention. In anyevent, we selectively place Cu posts 1408 on only some of the pads 1404in order to provide negative power interconnects to help reduce oreliminate electromigration. We have found that reducing the number of Cuposts on the substrate reduces the total stress on BEOL during the flipchip assembly process thereby reducing the failure rate in joiningwafers to organic substrates.

In one embodiment we provide a method comprising forming a semiconductorflip chip from a wafer having solderable electrical conducting sites anda substrate having electrical connecting pads and electricallyconductive posts operatively associated with the pads and extending awayfrom the pads to terminate in distal ends, wherein the posts are placedonly on some of the pads and solder bumps placed on at least some of theother of the pads that do not have the posts in order to decrease anystress in the flip chip, the steps comprising solder bumping the distalends through openings in a solder mask by injection molding solder ontothe distal ends to produce a solder bumped substrate and soldering thesolder bumped substrate to the sites wherein the distal ends extend intothe mask through the openings. In other embodiments the height of theposts may be substantially the same as the thickness of the mask or lessthan the thickness of the mask.

The foregoing method may further comprise positioning the solder bumpedsubstrate on the wafer in a manner that leaves a gap between the waferand the substrate and applying a no-flow underfill encapsulant layerbetween the wafer and the substrate to adhere to the wafer and thesubstrate.

Throughout this specification, abstract of the disclosure, and in thedrawings the inventors have set out equivalents, including withoutlimitation, equivalent elements, materials, compounds, compositions,conditions, processes, structures and the like, and even though set outindividually, also include combinations of these equivalents such as thetwo component, three component, or four component combinations, or moreas well as combinations of such equivalent elements, materials,compositions conditions, processes, structures and the like in anyratios or in any manner.

Additionally, the various numerical ranges describing the invention asset forth throughout the specification also includes any combination ofthe lower ends of the ranges with the higher ends of the ranges, and anysingle numerical value, or any single numerical value that will reducethe scope of the lower limits of the range or the scope of the higherlimits of the range, and also includes ranges falling within any ofthese ranges.

The terms “about,” “substantial,” or “substantially” as applied to anyclaim or any parameters herein, such as a numerical value, includingvalues used to describe numerical ranges, means slight variations in theparameter. In another embodiment, the terms “about,” “substantial,” or“substantially,” when employed to define numerical parameter include,e.g., a variation up to five per-cent, ten per-cent, or 15 per-cent, orsomewhat higher.

All scientific journal articles and other articles, including internetsites, as well as issued and pending patents that this writtendescription mentions including the references cited in such scientificjournal articles and other articles, including internet sites, and suchpatents, are incorporated herein by reference in their entirety and forthe purpose cited in this written description and for all otherdisclosures contained in such scientific journal articles and otherarticles, including internet sites as well as patents and the aforesaidreferences cited therein, as all or any one may bear on or apply inwhole or in part, not only to the foregoing written description, butalso the following claims, abstract of the disclosure, and appendeddrawings.

Although the inventors have described their invention by reference tosome embodiments, other embodiments defined by the doctrine ofequivalents are intended to be included as falling within the broadscope and spirit of the foregoing written description, and the followingclaims, abstract of the disclosure, and appended drawings.

We claim:
 1. In a method comprising forming a semiconductor flip chipfrom a wafer having solderable electrical conducting sites and asubstrate having electrical connecting pads and electrically conductiveposts operatively associated with said pads and extending away from saidpads to terminate in distal ends, wherein said posts are placed only onsome of said pads and solder bumps placed on at least some of the otherof said pads that do not have said posts in order to decrease any stressin said flip chip, the steps comprising solder bumping said distal endsthrough openings in a solder mask by injection molding solder onto saiddistal ends to produce a solder bumped substrate and soldering saidsolder bumped substrate to said sites wherein said distal ends extendinto said mask through said openings.
 2. The method of claim 1 where theheight of said posts is substantially the same as the thickness of saidmask or less than the thickness of said mask.
 3. The method of claim 1further comprising positioning said solder bumped substrate on saidwafer in a manner that leaves a gap between said wafer and saidsubstrate and applying a no-flow underfill encapsulant layer betweensaid wafer and said substrate to adhere to said wafer and saidsubstrate.